The present invention relates to a fuel injection system for an internal combustion engine.
The present invention concerns fuel injection systems of internal combustion engines, in particular systems for injection of fuel directly into combustion cylinders of compression ignition engines. In particular, it concerns fuel injection systems featuring a control valve for pressure relief in the nozzle of the injector. Such solutions are typically applied in common rail injection systems for preventing a leakage of fuel through the closed nozzle, which is otherwise difficult to avoid when using low viscosity fuels such as DME.
An example of such a prior art system is shown in FIG. 1. In that system, there are automatic isolating valves for preventing leakage of fuel through the closed nozzles from the fuel supply system to the engine combustion chambers during system standby on a non-operating engine. By joining the return lines of a set of the injectors into a common line and then connecting that common line to the isolating valve, the total number of the isolating valves is kept low. There is however a disadvantage in this design which is the relatively big number of hydraulic connection ports in the injector.
Another disadvantage is a relatively big dead volume of the isolating valve which is added to the high-pressure volume confined between the control valve 10 and the nozzle 11. The bigger that dead volume, the bigger is the control leakage and the worse controllability due to delays associated with building up and relieving pressure in that volume.
In another example of a prior art system shown in FIG. 2, the number of connection ports in the injector is reduced from three to two, but at the expense of the increased number of the automatic isolating valves, two per injector.
A bigger number of either the hydraulic connection ports and/or isolating valves deteriorates reliability of the system and increases its cost. The present invention is intended to improve reliability, reduce cost, improve controllability and reduce leakage of the prior art systems.
Another issue with the prior art systems is a relative difficulty in controlling the nozzle opening pressure (NOP). It can be controlled by means of a pressure regulator installed between the spring chambers of the set of injectors and the return conduit, as shown in FIG. 3, which again implies three connection ports per injector. By providing a relatively large volume of the nozzle spring chamber, it is in principle possible to eliminate the NOP control port and have a pressure regulator connected between the return line and the return conduit as shown in FIG. 4, but in that case the NOP control is complicated by differences in the leakages along the nozzle needle guides of different injector samples, the influence of the residual pressure on the nozzle closing pressure and leakage past the closed nozzle, and is besides relatively slow-acting. The present invention also offers means of improving the NOP control in such fuel injection systems.
It is desirable to provide a fuel injection system with reduced complexity, improved energy efficiency and better controllability of injection rate.
The fuel injection system according to an aspect of the present invention incorporates a fuel tank, a feed pump and associated components forming a low-pressure system, and a high-pressure pump delivering fuel under pressure into a common rail, which supplies pressurised fuel to all injectors of a multi-cylinder engine. A first automatic isolating valve is installed between the common rail and the injector, which incorporates a three-way electrically operated pilot valve that controls a hydraulically operated valve positioned between the common rail and a nozzle, and an electrically operated, two-way, normally open spill valve positioned between the outlet of the hydraulically operated valve and a return line. The nozzle has a needle that is biased by a return spring towards closing the nozzle. The return spring is installed in a spring chamber which, if pressurised, can assist the spring in biasing the needle towards nozzle closing. The spring chamber, the outlet of the pilot valve and the outlet of the spill valve are connected to an injector return line.
The return lines of the injectors are joined together into a single return conduit, which is connected via a second automatic isolating valve to the low-pressure system. A restriction is placed between the return line of each injector and the return conduit.
Installing the first automatic isolating valve between the common rail and the injector instead of installing it between the outlet of the hydraulically operated valve and the nozzle, as in the prior art systems, allows to reduce the dead volume upstream of the nozzle, which is drained between the injections, and by this means improve controllability and increase hydraulic efficiency of the injection system. Another advantage of this is a simplified design, improved reliability and reduced cost of the fuel injection system, because the injector can have only two connection ports, high pressure and return, and at the same time the total number of automatic isolating valves can be kept to a minimum equal to the number of injectors plus one. In prior art systems, the total number of the automatic isolating valves should be double the number of injectors if the latter have two ports. Otherwise, if a common automatic isolating valve is used in the return conduit of a prior art system, the number of ports on the injector must be increased to three.
Placing a restriction between the return line and the return conduit makes the fuel injection system adapted for control of the nozzle opening pressure, which can also be exercised individually for each injector and injection cycle and does not require additional pressure regulator as in the prior art systems.